In the prior art, among techniques for rewriting software saved in a memory, there is a technique for a software supply device to receive from a wireless terminal device version information of control software of the wireless terminal device, determine whether update is necessary, and download new control software to the wireless terminal device when update is necessary (For example, see Unexamined Japanese Patent Application Kokai Publication No. 2001-78258).
In the software update as disclosed in the above patent literature, an attempt to rewrite software of a portable electronic device having a relatively small battery capacity such as an electronic wristwatch has the risk of data rewriting operation being cut off before the operation is completed depending on the remaining battery level or the state of the electronic device such as the temperature of the electronic device.